This invention relates generally to bus-effected digital communications systems and more specifically to a digital interface circuit for neutralizing spurious electrical noise and to facilitate digital communications between a system controller coupled by a communication bus to remotely located, controlled apparatus. The interface circuit is particularly useful in certain types of heating, ventilating and air conditioning (HVAC) process control systems.
Bus-linked process control systems typically include a system controller coupled to one or more sensors for receiving signals therefrom relative to the valves of process parameters. Such a controller will process the received signals and depending upon the values thereof, generate command signals for maintaining or modifying the parameters. Such command signals are directed to remotely-located equipment operative to control such parameters. Additionally, it is frequently desirable for the remote control equipment itself to generate signals to be directed to the system controller along the linking bus. With the advent of microprocessor-based low voltage digital communications networks which form a part of such process control systems, it has been found that spurious electrical noise which may appear on the linking bus will, unless addressed by the system designer, result in the transmission of false digital signals between the controller and the remote equipment.
Spurious electrical noise may comprise one or more types as, for example, a persistent spurious noise which has a maximum half-wave amplitude substantially less than the maximum amplitude of the digital signal. Another type of unwanted noise may result from any precipitous, spurious depression of the switched bus voltage for a duration insufficient to represent a valid change in logic state and yet which reaches a value otherwise indicative of such a change. Yet another type of unwanted electrical noise may include random transient voltage spikes having a maximum amplitude greater than that of the digital signal and yet have a duration substantially less than one bit time. Further, these transient voltage spikes may exhibit a polarity corresponding to or opposite from the polarity of the digital signal.
One approach to the neutralization of certain types of spurious noise is to utilize a two-wire, ungrounded bus and generating digital signals thereon by voltages of substantially equal magnitude and of alternating, opposite polarity one from the other as, for example, by switching between +12 V. and -12 V. Authentic digital signals generated by switching between two such voltages are readily distinguishable from low amplitude noise which may appear in the region of 0 V. Bus arrangements of this type attempt to eliminate interference from transient spikes having relatively high magnitudes by using bus shielding and/or other special wiring techniques. Yet another approach to the neutralization of unwanted noise is to utilize a two-wire grounded bus and generate digital signals by switching the bus between a first voltage, either positive or negative, and 0 V. A logic midpoint is selected to be nominally equal to one half of the first voltage and if the value of the digital voltage lies above this midpoint, it is presumed to be at logic "1" and if below, at logic "0". With this approach, a low-amplitude spurious signal which may occur about the midpoint or which may be superimposed upon the digital signal itself may likewise cause false signals unless steps such as extensive noise filtering or the provision of elaborate message checking sequences are used. Additionally, these approaches fail to appreciate the manner in which the interfacing circuit may incorporate means for resetting the circuit and may be protected from the inadvertent application of unwanted, steady state voltages which may result from, for example, an incorrect wiring connection.
A noise-neutralizing digital interface circuit to facilitate bidirectional communications and which is capable of distinguishing between authentic digital signals and unwanted, low level spurious noise, which is capable of neutralizing a high amplitude noise of brief duration, which is capable of attenuating a brief, precipitous voltage depression on the switched bus, which includes means for protecting the circuit from the damaging effects of undesired transient or steady state voltages and which may optionally include a reset function when the interface circuit is used on remotely-located, controlled devices would be a significant advance over the prior art.